Coated abrasive article and method of making



i atenteci Sept. 2,

' COATED ABRASIVE ARTICLE AND METHOD OF MAKING Leonard B. Nestor, St.Paul, Minn assignor to a Minnesota Mining & Manufacturing Company, St.Paul, Minn, a corporation of Delaware No Drawing. Application June 28,1947, Serial No. 757,929

7 Claims. (Cl. 51-302) This invention relates to abrasive coatedarticles and methods of making. In particular, it is concerned with theproduction of sandpaper or abrasive sheet material wherein the abrasivegrit is bonded to the supporting backing by a binder compositioncomprising a modified starch ad-. hesive.

Coated abrasive sheet material, such as sandpaper, comprises generally abase of flexible sheet material such as paper or cloth, a more or lessfinely comminuted resistant material having abrading properties, such asgarnet, flint, corundum or the like, and a binder, generally in the formof a glue or resinous bond, which serves to bindjthe'abrasive particlesto the base. The binder is conveniently applied in two separatecoatings, one before and one after the application of the abrasivegrains, i. e. as abrasive bond and abrasive size coats, and isultimately cured by heating or drying.

Various modifications of starch have in the past been successfullyemployed as adhesives for many purposes; For bonding plywood, or sizingcloth,

or other similar applications, these prior art starch adhesives havebeen quite, satisfactory;

The requirements of adhesives for use in the coated abrasive art are,however, much more severe. Iheuse of starch adhesives in the bonding ofabrasivegritsto flexible backings, for the production of sandpaper, hasnot previously been successfully attempted, as far as I am aware.

An outstanding requirement of adhesives designed for the manufacture ofcoated abrasive products-is high bonding strength. This term,

cosity, even at high concentration, usually indi-' cates a weakadhesive, and frequently causes dimculty from flowing of the adhesive onthe coated sheet.

' In producing a wide variety of coated abrasive products, with varioustypes and sizes of abrasive grains, many different adhesive compositionsare required. Thus, the abrasive bond composition 2 V is customarilymore viscous and more concentrated than the sizing'composition.Additives or extenders, such as inert powders or the like, may

be introduced for one purpose or another, and

will have an efiect on both solids content and viscosity. The amount ofbinder, and hence its viscosity and solids content, required foraspecific coated abrasive article will depend, among other I things, onthe grit size of the abrasive grains... The nature of the paper, clothor other backing material must also be taken into consideration indefining the properties or" the binder prior tojits application. Thetemperature of application may vary within quite Wide limits. Adhesiveswhich, meet therequirements as to solids content and" spreadability orcoatability after being thus die luted, compounded, heated, or otherwiseprepared for use in the making of sandpaper are found to. come withinthe approximate limits of 1000 to 8000 centipoise viscosity and 40-50percent solids when measured at 45 C. I produce by my inventioncompositions which, in addition to coming within this viscosity andconcentration range, are fully adequate both in adhesion to abrasivegrit and in internal strength and toughness of the dried film, and aretherefore fully suited to the manufacture of coated abrasive articles ofthe nature of sandpaper. N

My invention additionally provides many other advantages. The processemployed may b carried out in a continuous operation, producing the Afinished'adhesive composition'directly from. the raw starch, or it maybe broken downinto indi vidual and separate steps for greaterflexibility if desired. The dried f adhesive film is tough and strong,and holds the abrasive grit firmly bonded to the backing under theintense strains encountered during abrading operations, yetissufiiciently flexible I01 all normal use. The coated abrasive sheetmaterial has superior resistance to grit loss during abradingoperations, and does not. deteriorate at elevated temperatures undertest conditions ornormal conditions of storage or use. The abrasivearticle. containsno proteinaceous components and is accordingly lesssusceptible to. the destructive action of certain'bacteria. By the useof amodified starch abrasive bond in accord .ance with this disclosure,there results an abrasive article having long life and durability, aswell as astrong granular bond. e I These and other advantages may beconvene iently obtained, according to my invention, by employing, asa'grit binder, adhesive compositions made by treating raw starch oranalogous material with an oxidizing agent to a predeter minedend-point, and subsequently converting the treated starch to a viscousfluid adhesive condition by the addition of an active base, e. g. sodiumhydroxide. Specific details of exemplary procedures will now be setforth in order more clearly to describe and illustrate the invention,but without any intention of being limited thereto. In the followingexamples, al1 parts are given as parts by weight unless otherwisespecified.

Example 1 To a suspension of 100 parts of potato starch in 114 parts ofwater at 100 F. was added parts of a solution of sodium hypochloriteanalyzing 14% available chlorine and having a specific grayity of about1.2. The mixture, which was in the form of a rather fluid slurry, wascontinuously stirred, and portions were removed periodically fordetermination of acidity and for further treatment. The initial mixturewas found to be slightly alkaline and to have a pH value ofapproximately '8.5-9.0; the pl-l rapidly decreased'as the reactionproceeded; the solution soon becom ins acidic.

"To a portion of the above acidic slurry, after determination of its pHvalue, was then added a quantityof "a 50% aqueous solution of sodium hydro'xide'sufficient to form a homogeneous solu tlfoii; The addition ofthe base immediately stops the"re'action between starch and oxidizingagent, and converts the reacted starch slurry to a smooth, viscoussolution of somewhat darker color. The viscosity at this stage is foundto be dependent both onthe extent of the reaction be tweer'i'starch andhypochlorite, as measured by the reduction in pH value of the slurry,and by the amount'of sodium hydroxide added in excess of thatamount'justsufiicient to produce a homogeneoussmooth solution. In the presentexample, 3.34parts of the'sodium hydroxide solution was added ,topartsof the reacted starch slurry to obtain adhesive compositionscontaining solids and having the following viscosities:

M I I R viscosity oi reaction H r dh Sample No. t P 0 a cave mmfitgsslurry is 6.65" ,95 25 6.18 4,400 5. O0 240 4.27 1,60

ift istfo benoted that the alkali is here present inthe ratio of12i5parts of allgali to parts of the "starch initially present, whereas onlyabout 10 parts of alkali was required to convert the mix ture'toasmooth, viscous and homogeneous solu; tion 7 The additional alkaliprovided a solution of somewhat improved fluidity and stability.

"Pc'irtions'of these adhesives were cast in film form. The dried filmsof samples 1 and 2, havi ng initial viscosities of at least about 3000cps.,'were found'to' be self-supporting, whereas the otherscrackefdor'broke apart during drying. However, it was fdulid that theaddition to the liquid ad-' h'esive of a powdered'inert filler; suchasg'r'ound limestone or'silica dust, improved the 'film-fo'rmingfabilities of the adhesive sufilcientlyso that self supporting filmscould be formed of modifled-starch adhesives having initial viscositiesas low as about 1000 cps. Finely ground'limestone filler was thereforeadded, in an amount equivalent'to 30-50% of the total volume of thedried film, to the adhesive compositions, which as em ployed in themanufacture of sandpaper were dry filler hesive water Abrasive 1555a til '32. Abrasive size coat 16.

Grit 1 flint paper (sandpaper carrying grit 1 flint abrasive grains) wasprepared using these'various adhesive compositions. The adhesives coatedequally as well as the conventional animal gluefcompositions. Theyrequired somewhat longerdrying time or higher drying temperature thanthe corresponding animal glue adhesives used in the same amount.

The finished abrasive sheet was tested by rubbing a weighed strip underconstant pressure against'a similar strip and determining the resultingunit loss in weight. Grit 1Y2 flint paper having an animal glue bond andsize coatis con; sidered satisfactorily to pass this "rub test" if theloss of weight is 3-5 percent or less. Using the described starch baseadhesives, theruldte'st values were as follows:

The sodium hypochlorite solution used in the above example may be'preparedby passing 'chlo'f rine into a solution ofsodium'-hydroxide',or in any other convenientmann'er. The strength ofthesolution, expressed in terms of-percent avail able chlorine, isconveniently determined'by ad'- dition of a portion to acidic iodidesolution and titration of the liberated iodine with standard thiosulfatesolution. For economical and safe operation, the solution should containapproxlmately the indicated" amount of available "chlorine, althoughother concentrations have also been found useful. Y The hypochloritesolution may be added to the starch slurry as'indi'cated', or may beadd'e'dto' the water before the addition of the starch: Iii the lattercase, the s'tarchshouldbe-mixed in ra'p idly so as to ensure equalreaction" dis-11" p'oi tions.

Various other oxidizing agents may be employed in place orhypochlo'rites'in'making my improved adhesive compositions and "coatedabrasive products, and the steps or undamaged conversion mayor may notbe carried out lii" a continuous process, as desired; The method "ofpreparation hereinabove described has, however.

been found highly efiective'whenappliedto the manufacture of coatedabrasive articles or the nature of sandpaper.

. E-mmnleZ In this example, acidic permanganate solution crease in theviscosityv of the alkali-converted adhesive. l

Sandpaper was prepared, using a permanganate-treated starch adhesive, bya process similar in general to that described under'Example l-. Apre-formed slurry of 2700 parts of potato starch in water containing 120parts of concentrated hydrochloric acid was treated with potassiumpermanganate, added in the form of a solution. At the completion of thereaction, at which point the purplish-brown color was completelydischarged, 960 parts of 50% sodium hydroxide solution'was added;approximately l00.parts was required to neutralize the hydrochloricacid, leaving about 15.9 parts of NaOH available for each 100 parts ofstarch initially present, of which about, parts was Just sufficient toprovide aviscosity, loss in Q F cps, rub test, Y 45 C percent Theoxidation of the starch has been successfully carried out attemperatures from 80 R, at which the reaction is quite slow, to l20-125E; at higher temperatures the starch is gelatinized.

Potato starch, specified in the examples, has been successfully replacedby other varieties of starch, e. g., corn starch, sago starch, and evenby certain modifications of cellulose. Air-dry starch normally contains.about 10% moisture, conveniently determined by azeotropic distillationwith'toluene by theBidwell-Sterlingmethod. Where the moisture contentdiffers significantly from this value, corresponding changes are made inthe weight of air-dry starch shown in the formulas given. 7

Sodium hydroxide is completely converting the mixture of treated starchand water to the viscous adhesive state. As previously noted, a minimumamount is required to thus convertall of the treated starch and toproduce a smooth homogeneous solution; small additional amounts thenrapidly reduce the viscosity of the solution. Even with these addedamounts of base, the adhesive does not appear to be particularlyirritating to the skin, nor deleterious to paper or other backingmaterials. The presence of much larger quantities of sodium hydroxide orthe like in the dried adhesive film causes'undesirable weakening of thefilm and is to be avoided. In general, amounts of base equivalent toabout 10-12 parts of sodium hydroxide for 100 parts of starch are foundto produce the best results, although 9 parts are ordinarily sufficientto convert the starch to a homogeneous solution, and up to about 16parts have produced useful adhesive compositions.

Potassium hydroxide, sodium metasilicate, or quaternary ammonium orsulfonium bases are equally as efiective as sodium hydroxide in thepreparation of the converted starch adhesive.

effective mama and ing my novel abrasive sheet material -maybe filled orcompounded with powdered calcium carbonate .or other inert filler, asshown, in' which case adhesives of somewhat reduced initial viscosity(i. e., viscosity-of the converted starch-solution just subsequent toaddition of sodium hydroxide or the like) andreduced film strengthmaysuccessfully. be employed. Adhesives having a higher initialviscosity, and correspondingly higher film strength, may be used asabrasive bond and abrasive size coatings without'the'addition of anyfiller, producing abrasive'coated articles having advantages oversimilar articles p'roduced with animal glue adhesives. ('Iheaddition ofsmall amounts of untreated starch to the adhesive has been considered asa means of increasing the viscosity, but such additions donotsignificantly increase thefilm strength.) The starch base adhesivemay also'be used inconjunction with other adhesives. For example, it maybe blended with glue, casein, sodium silicate, etc.; or it may be usedas either-the abrasive bond coat or the sizing coat in conjunction withother known bonds or sizings'. Thus, sandpaper having a modified starchabrasive bond may be sized with animal glue or with synthetic 'resin; Inthe latter case, the alkali contentof'the bond coat may serve as acatalyst to increase the rate of gelation or hardening ofapropilyiselectedsyhthetic resin size coat, e. gaa size coat comprisinga re'sorcinol-aldehyde or other phenol'aldehy'de resin. In all cases,abrasive coated sheet material is produced which has superior propertiesheretofore considered impossible of attainment with starch base adhesivecompositions."

WhatIclaim is: a v 1. A flexible abrasive article of the nature ofsandpaper, having abrasive grits adhesively bonded to a flexible backingby a binder comprising the driedresidue of a distinctly alkaline;--c0ncentrated spreadable viscous adhesive solution of alkaline convertedoxidized starch, said solution at a concentration of about 40-50%,solids having a viscosity of about 1000-8000 centipoises when measuredat 45 C, and being prepared by reacting together, at a temperature of atleast about 80 F. but less than that required for gelatinization of thestarch, and to a moderately acid pH value, a concentrated aqueous slurryof 100 parts of starch and 10 parts of a sodium hypochlorite solution of1.2 specific gravity and 14% available chlorine, and then terminatingthe reaction and simultaneously converting the mass to said distinctlyalkaline adhesive solution by adding 9-16 parts of sodium hydroxide.

2. A flexible abrasive article of the nature of sandpaper, havingabrasive grits adhesively bonded to a flexible backing by a bindercomprising the dried residue of a concentrated spreadable viscousadhesive solution of alkaline converted oxidized starch, said solutionat a concentration of about 40-50% solids having a viscosity of about1000-8000 centipoises when measured at 45 C. and being prepared :bymixing starch and a watersoluble oxidizing agent in water and oxidizing1- the-. starch, at; a temperature of:- at: least. about 802 E: but:less; thani thatrequired: for gei'atinie zatiom or; the starch, andwithout. substantially, reducing. itsinternalz strength, to l apnecleterm-ined amt-pointat; a; moderatelyacicl pH{ value, and; thenneutralizing: the acid; anal converting" the mass; to. a; distinctlyalkaline, homogeneous visa-- cous fluid adhesive mass by-addinganactiveial kali-ifrqm thenclassconsistingofa thealkali metal hydroxidesand; sodium metasilicate, in. an amount: equivalent 1210:9461 parts of;sodiurm hydroxide for 1:00- parts-of unoxidized; starch.

3-; 'Ihe: product-of clai1n2=in whichtbewatersolublegoxidizingagent; isa. hypochloritcl.

4; The product; of claim 2zin, which; the-watersolubleoxidizingiagentis-a permanganate.

5;. A flexible abrasive; article, of. thev nature of; sandpaper, having;abrasive grits. adhesively; hqnclcoi. tQa a flexible. backin a bindercome prising, the dried; residues of; a: concentrated sgreadable;viscous adhesive solution ofrallialine converted oxidized starch, saidsolutionla-t a: con centration of about 4075.096, solids-having a-.vis--cosity; o,-about;.1000,-8 000 centipoises when meas uI$i-;al;; 45?-C.and -bei ng,. preparedvby reacting to-- gether, ata; temperaturenof atleast about-. F3 but; less I than; thatrequired a for,- gelatinization;of; the; starch; and; at a moderately: acid-1 DH value;v a;concentrated; aqueous slurry; of- 2100- parts 2 of tar ha miiah utafi;pa ts of potass um-perma w ganate; and thenneutralizin a theacid; andcon-- verting: the; oxidized starch to at distinctlyl alkae ice .o o cnus v scousaflu d'adhesivemass-bv agld n 9; 6 parts; of'soclium hydroxideforeach: lllllpar ofgthe unoxidized-starch;V

6 A; flexible: abrasive article of: the; nature. of ane ea crn havingbrasive a itsr adhesivelr- -Qnd e: o;.a: cxib cki an a. grit-.1bonding;- coat --a n cl a; surfacesizling coat, saidi gritwbcnding:coat; comprising the alkalieconverted', oxidized starch product; formed:by oxidizing starch, at: a temperatureqof. at least about .80F!.-butless than thatrequiredfor'gelatinization of-Uthe starch; to: apredetermined end-pointlat r a. moderately. acid pH value withoutsubstantially: reducing itszinternal strength, and converting; to a.distinctly alkaline,- viscous; fluid adhesive: massof;- highbondingstrength, havinga: viscosity of'about' 100043000 centipoises at aconcentration of about;

4 k%-solids when measured-at C., byia-ddingc metals hydroxides: and;sodiummetasihcate .im an amount equivalent to 9-16 parts of sodium: 11y.dIQXidE; for each. 100 parts' ofl unoxidizezilstarch, and: said:surface: sizing? comprising; an; alkali? catalyzed synthetic: phenolicresin;

7; As. coated abrasive sheet; material of: the: nae ture. of; sandpaper;having abrasive grits;v adher sivcly" bonded. to azfiexible; backing: bya: binder; composition comprisingthe alkalieconvertedifixidizedzxrstarch product formed by oxidizing. starch ataa;temperature, of. at. least about; but less: than, thatrequired rorgelatinization of. the starch, to. a; predeterminedend-point at. a:moderately aciollpH' value Without. substantially, 1 reducing its;internal strength, and converting: to: a: distinctly: alkaline; viscousfluidadhesive; mass: havingral viscosity of; about: 1000-8000.:centipoiscs; ata concentration-bf about 409-5095.; solids: when:measuredaatfi? C.-, .by;-adding;an activezbaseifromz the; class;consisting; of alkali; metal. hydroxidesand sodium metaosilicatesin anamounti equivalent to;-9,-l6 arts: or sodi-um ;hydroxide:for: parts of zthe:unoizidizedstarch.

LEONARD R. NESTOR;

-REFERENGES1 erran The following references are of record in the file ofthis patent:

UNITED STATES PATENTS OTI-IEI-c- REFERENCES 1? Kerr, Starch" (1944), pp.235-239

6. A FLEXIBLE ABRASIVE ARTICLE OF THE NATURE OF SANDPAPER, HAVINGABRASIVE GRITS ADHESIVELY BONDED TO A FLEXIBLE BACKING BY A GRIT BONDINGCOAT AND A SURFACE SIZING COAT, AND GRIT BONDING COAT COMPRISING THEALKALI-CONVERTED OXIDIZED STARCH PRODUCT FORMED BY OXIDIZING STARCH AT ATEMPERATURE OF AT LEAST ABOUT 80* F. BUT LESS THAN THAT REQUIRED FORGELATINIZATION OF THE STARCH, TO A PREDETERMINNED END-POINT AT AMODERATELY ACID PH VALUE WITHOUT SUBSTANTIALLY REDUCINNG ITS INTERNALSTRENGTH, AND CONVERTING TO A DISTINCTLY ALKALINE, VISCOUS FLUIDADHESIVE MASS OF HIGH BONDING STRENGTH, HAVING A VISCOSITY OF ABOUT1000-8000 CENTIPOISES AT A CONCENTRATION OF ABOUT 40-500% SOLIDS WHENMEASURED AT 45* C., BY ADDING AN ACTIVE BASE FROM THE CLLASS CONSISTINGOF ALKALI METAL HYDROXIDES AND SODIUM METASILICATE IN AN AMOUNTEQUIVALENT TO 9-16 PARTS OF SODIUM HYDROXIDE FOR EACH 100 PARTS OFUNOXIDIZED STARCH, AND SAID SURFACE SIZING COMPRISING AN ALKALICATALYZEDSYNTHETIC PHENOLIC RESIN